In the last decade the concept of developing anti-cancer medications which target abnormally active protein kinases has led to a number of successes. In addition to the actions of protein kinases, lipid kinases also play an important role in generating critical regulatory second messengers. The PI3K family of lipid kinases generates 3′-phosphoinositides that bind to and activate a variety of cellular targets, initiating a wide range of signal transduction cascades (Vanhaesebroeck et al., 2001; Toker, 2002; Pendaries et al., 2003; Downes et al., 2005). These cascades ultimately induce changes in multiple cellular processes, including cell proliferation, cell survival, differentiation, vesicle trafficking, migration, and chemotaxis.
PI3Ks can be divided into three distinct classes based upon differences in both structure, and substrate preference. While members of the Class II family of PI3Ks have been implicated in the regulation of tumor growth (Brown and Shepard, 2001; Traer et al., 2006), the bulk of research has focused on the Class I enzymes and their role in cancer (Vivanco and Sawyers, 2002; Workman, 2004, Chen et al., 2005; Hennessey et al., 2005; Stauffer et al., 2005; Stephens et al., 2005; Cully et al., 2006).
Class I PI3Ks have traditionally been divided into two distinct sub-classes based upon differences in protein subunit composition. The Class IA PI3Ks are comprised of a catalytic p110 catalytic subunit (p110α, (3 or 6) heterodimerized with a member of the p85 regulatory subunit family. In contrast, the Class IB PI3K catalytic subunit (p110γ) heterodimerizes with a distinct p101 regulatory subunit (reviewed by Vanhaesebroeck and Waterfield, 1999; Funaki et al., 2000; Katso et al., 2001). The C-terminal region of these proteins contains a catalytic domain that possesses distant homology to protein kinases. The PI3Kγ structure is similar to Class IA p110s, but lacks the N-terminal p85 binding site (Domin and Waterfield, 1997). Though similar in overall structure, the homology between catalytic p110 subunits is low to moderate. The highest homology between the PI3K isoforms is in the kinase pocket of the kinase domain.
The Class IA PI3K isoforms associate with activated receptor tyrosine kinases (RTKs) (including PDGFR, EGFR, VEGFR, IGF1-R, c-KIT, CSF-R and Met), or with tyrosine phosphorylated adapter proteins (such as Grb2, Cbl, IRS-1 or Gab1), via their p85 regulatory subunits resulting in stimulation of the lipid kinase activity. Activation of the lipid kinase activity of the p110β and p110γ isoforms has been shown to occur in response to binding to activated forms of the ras Oncogene (Kodaki et al, 1994). In fact, the oncogenic activity of these isoforms may require binding to ras (Kang et al., 2006). In contrast, the p110α and p110δ isoforms exhibit oncogenic activity independent of ras binding, through constitutive activation of Akt.
Class I PI3Ks catalyze the conversion of PI(4,5)P2 [PIP2] to PI(3,4,5)P3 [PIP3]. The production of PIP3 by PI3K affects multiple signaling processes that regulate and coordinate the biological end points of cell proliferation, cell survival, differentiation and cell migration. PIP3 is bound by Pleckstrin-Homology (PH) domain-containing proteins, including the phosphoinositide-dependent kinase, PDK1 and the Akt proto-oncogene product, localizing these proteins in regions of active signal transduction and also contributing directly to their activation (Klippel et al., 1997; Fleming et al., 2000; Itoh and Takenawa, 2002; Lemmon, 2003). This co-localization of PDK1 with Akt facilitates the phosphorylation and activation of Akt.
Carboxy-terminal phosphorylation of Akt on Ser473 promotes phosphorylation of Thr308 in the Akt activation loop (Hodgekinson et al., 2002; Scheid et al., 2002; Hresko et al., 2003). Once active, Akt phosphorylates and regulates multiple regulatory kinases of pathways that directly influence cell cycle progression and cell survival.
Many of the effects of Akt activation are mediated via its negative regulation of pathways which impact cell survival and which are commonly dysregulated in cancer. Akt promotes tumor cell survival by regulating components of the apoptotic and cell cycle machinery. Akt is one of several kinases that phosphorylate and inactivate pro-apoptotic BAD proteins (del Paso et al., 1997; Pastorino et al., 1999). Akt may also promote cell survival through blocking cytochrome C-dependent caspase activation by phosphorylating Caspase 9 on Ser196 (Cardone et al., 1998).
Akt impacts gene transcription on several levels. The Akt-mediated phosphorylation of the MDM2 E3 ubiquitin ligase on Ser166 and Ser186 facilitates the nuclear import of MDM2 and the formation and activation of the ubiquitin ligase complex. Nuclear MDM2 targets the p53 tumor suppressor for degradation, a process that can be blocked by LY294002 (Yap et al., 2000; Ogarawa et al., 2002). Downregulation of p53 by MDM2 negatively impacts the transcription of p53-regulated pro-apoptotic genes (e.g. Bax, Fas, PUMA and DR5), the cell cycle inhibitor, p21Cip1, and the PTEN tumor suppressor (Momand et al., 2000; Hupp et al., 2000; Mayo et al., 2002; Su et al., 2003). Similarly, the Akt-mediated phosphorylation of the Forkhead transcription factors FKHR, FKHRL and AFX (Kops et al., 1999; Tang et al., 1999), facilitates their binding to 14-3-3 proteins and export from the cell nucleus to the cytosol (Brunet et al., 1999). This functional inactivation of Forkhead activity also impacts pro-apoptotic and pro-angiogenic gene transcription including the transcription of the Fas ligand (Ciechomska et al., 2003) Bim, a pro-apoptotic Bcl-2 family member (Dijkers et al., 2000), and the Angiopoietin-1 (Ang-1) antagonist, Ang-2 (Daly et al., 2004). Forkhead transcription factors regulate the expression of the cyclin-dependent kinase (Cdk) inhibitor p27Kip1. Indeed, PI3K inhibitors have been demonstrated to induce p27Kip1 expression resulting in Cdk1 inhibition, cell cycle arrest and apoptosis (Dijkers et al., 2000). Akt is also reported to phosphorylate p21Cip1 on Thr145 and p27Kip1 on Thr157 facilitating their association with 14-3-3 proteins, resulting in nuclear export and cytoplasmic retention, preventing their inhibition of nuclear Cdks (Zhou et al., 2001; Motti et al., 2004; Sekimoto et al., 2004). In addition to these effects, Akt phosphorylates IKK (Romashkova and Makarov, 1999), leading to the phosphorylation and degradation of IκB and subsequent nuclear translocation of NFκB, resulting in the expression of survival genes such as IAP and Bcl-XL.
The PI3K/Akt pathway is also linked to the suppression of apoptosis through the JNK and p38MAPK MAP Kinases that are associated with the induction of apoptosis. Akt is postulated to suppress JNK and p38MAPK signaling through the phosphorylation and inhibition of two JNK/p38 regulatory kinases, Apoptosis Signal-regulating Kinase 1 (ASK1) (Kim et al., 2001: Liao and Hung, 2003; Yuan et al., 2003), and Mixed Lineage Kinase 3 (MLK3) (Lopez-Ilasaca et al., 1997; Barthwal et al., 2003; Figueroa et al., 2003). The induction of p38MAPK activity is observed in tumors treated with cytotoxic agents and is required for those agents to induce cell death (reviewed by Olson and Hallahan, 2004). Thus, inhibitors of the PI3K pathway may promote the activities of co-administered cytotoxic drugs.
An additional role for PI3K/Akt signaling involves the regulation of cell cycle progression through modulation of Glycogen Synthase Kinase 3 (GSK3) activity. GSK3 activity is elevated in quiescent cells, where it phosphorylates cyclin D1 on Ser286, targeting the protein for ubiquitination and degradation (Diehl et al., 1998) and blocking entry into S-phase. Akt inhibits GSK3 activity through phosphorylation on Ser9 (Cross et al., 1995). This results in the elevation of Cyclin D1 levels which promotes cell cycle progression. Inhibition of GSK3 activity also impacts cell proliferation through activation of the wnt/beta-catenin signaling pathway (Abbosh and Nephew, 2005; Naito et al., 2005; Wilker et al., 2005; Kim et al., 2005; Segrelles et al., 2006). Akt mediated phosphorylation of GSK3 results in stabilization and nuclear localization of the beta-catenin protein, which in turn leads to increased expression of c-myc and cyclin D1, targets of the beta-catenin/Tcf pathway.
Although PI3K signaling is utilized by many of the signal transduction networks associated with both oncogenes and tumor suppressors, PI3K and its activity have been linked directly to cancer. Overexpression of both the p110α and p110β isoforms has been observed in bladder and colon tumors and cell lines, and overexpression generally correlates with increased PI3K activity (Bénistant et al., 2000). Overexpression of p110α has also been reported in ovarian and cervical tumors and tumor cell lines, as well as in squamous cell lung carcinomas. The overexpression of p110α in cervical and ovarian tumor lines is associated with increased PI3K activity (Shayesteh et al., 1999; Ma et al., 2000). Elevated PI3K activity has been observed in colorectal carcinomas (Phillips et al., 1998) and increased expression has been observed in breast carcinomas (Gershtein et al., 1999).
Over the last few years, somatic mutations in the gene encoding p110α (PIK3CA) have been identified in numerous cancers. The data collected to date suggests that PIK3CA is mutated in approximately 32% of colorectal cancers (Samuels et al., 2004; Ikenoue et al., 2005), 18-40% of breast cancers (Bachman et al., 2004; Campbell et al., 2004; Levine et al., 2005; Saal et al., 2005; Wu et al., 2005), 27% of glioblastomas (Samuels et al., 2004; Hartmann et al., 2005, Gallia et al., 2006), 25% of gastric cancers (Byun et al., 2003; Samuels et al., 2004; Li et al., 2005), 36% of hepatocellular carcinomas (Lee et al., 2005), 4-12% of ovarian cancers (Levine et al., 2005; Wang et al., 2005), 4% of lung cancers (Samuels et al., 2004; Whyte and Holbeck, 2006), and up to 40% of endometrial cancers (Oda et al., 2005). PIK3CA mutations have been reported in oligodendroma, astrocytoma, medulloblastoma, and thyroid tumors as well (Broderick et al., 2004). Based upon the observed high frequency of mutation, PIK3CA is one of the two most frequently mutated genes associated with cancer, the other being K-ras. More than 80% of the PIK3CA mutations cluster within two regions of the protein, the helical (E545K) and catalytic (H1047R) domains. Biochemical analysis and protein expression studies have demonstrated that both mutations lead to increased constitutive p110α catalytic activity and are in fact, oncogenic (Bader et al., 2006; Kang et al., 2005; Samuels et al., 2005; Samuels and Ericson, 2006). Recently, it has been reported that PIK3CA knockout mouse embryo fibroblasts are deficient in signaling downstream from various growth factor receptors (IGF-1, Insulin, PDGF, EGF), and are resistant to transformation by a variety of oncogenic RTKs (IGFR, wild-type EGFR and somatic activating mutants of EGFR, Her2/Neu) (Zhao et al., 2006).
Functional studies of PI3K in vivo have demonstrated that siRNA-mediated downregulation of p110β inhibits both Akt phosphorylation and HeLa cell tumor growth in nude mice (Czauderna et al., 2003). In similar experiments, siRNA-mediated downregulation of p110β was also shown to inhibit the growth of malignant glioma cells in vitro and in vivo (Pu et al., 2006). Inhibition of PI3K function by dominant-negative p85 regulatory subunits can block mitogenesis and cell transformation (Huang et al., 1996; Rahimi et al., 1996). Several somatic mutations in the genes encoding the p85α and p85β regulatory subunits of PI3K that result in elevated lipid kinase activity have been identified in a number of cancer cells as well (Janssen et al., 1998; Jimenez et al., 1998; Philp et al., 2001; Jucker et al., 2002; Shekar et al., 2005). Neutralizing PI3K antibodies also block mitogenesis and can induce apoptosis in vitro (Roche et al., 1994: Roche et al., 1998; Bénistant et al., 2000). In vivo proof-of-principle studies using the PI3K inhibitors LY294002 and wortmannin, demonstrate that inhibition of PI3K signaling slows tumor growth in vivo (Powis et al., 1994; Shultz et al., 1995; Semba et al., 2002; Ihle et al., 2004).
Overexpression of Class I PI3K activity, or stimulation of their lipid kinase activities, is associated with resistance to both targeted (such as imatinib and tratsuzumab) and cytotoxic chemotherapeutic approaches, as well as radiation therapy (West et al., 2002; Gupta et al., 2003; Osaki et al., 2004; Nagata et al., 2004; Gottschalk et al., 2005; Kim et al., 2005). Activation of PI3K has also been shown to lead to expression of multidrug resistant protein-1 (MRP-1) in prostate cancer cells and the subsequent induction of resistance to chemotherapy (Lee et al., 2004).
The importance of PI3K signaling in tumorigenesis is further underscored by the findings that the PTEN tumor suppressor, a PI(3)P phosphatase, is among the most commonly inactivated genes in human cancers (Li et al., 1997, Steck et al., 1997; Ali et al., 1999; Ishii et al., 1999). PTEN dephosphorylates PI(3,4,5)P3 to PI(4,5)P2 thereby antagonizing PI3K-dependent signaling. Cells containing functionally inactive PTEN have elevated levels of PIP3, high levels of activity of PI3K signaling (Haas-Kogan et al., 1998; Myers et al., 1998; Taylor et al., 2000), increased proliferative potential, and decreased sensitivity to pro-apoptotic stimuli (Stambolic et al., 1998). Reconstitution of a functional PTEN suppresses PI3K signaling (Taylor et al., 2000), inhibits cell growth and re-sensitizes cells to pro-apoptotic stimuli (Myers et al., 1998; Zhao et al., 2004). Similarly, restoration of PTEN function in tumors lacking functional PTEN inhibits tumor growth in vivo (Stahl et al., 2003; Su et al., 2003; Tanaka and Grossman, 2003) and sensitizes cells to cytotoxic agents (Tanaka and Grossman, 2003).
The class I family of PI3Ks clearly plays an important role in the regulation of multiple signal transduction pathways that promote cell survival and cell proliferation, and activation of their lipid kinase activity contributes significantly to the development of human malignancies. Furthermore, inhibition of PI3K may potentially circumvent the cellular mechanisms that underlie resistance to chemotherapeutic agents. A potent inhibitor of Class I PI3K activities would therefore have the potential not only to inhibit tumor growth but to also sensitize tumor cells to pro-apoptotic stimuli in vivo.
Signal transduction pathways originating from chemoattractant receptors are considered to be important targets in controlling leukocyte motility in inflammatory diseases. Leukocyte trafficking is controlled by chemoattractant factors that activate heterotrimeric GPCRs and thereby trigger a variety of downstream intracellular events. Signal transduction along one of these pathways that results in mobilization of free Ca2+, cytoskelatal reorganization, and directional movement depends on lipid-derived second messengers produced by PI3K activity (Wymann et al., 2000; Stein and Waterfield, 2000).
PI3Kγ modulates baseline cAMP levels and controls contractility in cells. Recent research indicates that alterations in baseline cAMP levels contributes to the increased contractility in mutant mice. This research, therefore, shows that PI3Kγ inhibitors would afford potential treatments for congestive heart failure, ischemia, pulmonary hypertension, renal failure, cardiac hypertrophy, atherosclerosis, thromboembolism, and diabetes.
PI3K inhibitors would be expected to block signal transduction from GPCRs and block the activation of various immune cells, leading to a broad anti-inflammatory profile with potential for the treatment of inflammatory and immunoregulatory diseases, including asthma, atopic dermatitis, rhinitis, allergic diseases, chronic obstructive pulmonary disease (COPD), septic shock, joint diseases, autoimmune pathologies such as rheumatoid arthritis and Graves' disease, diabetes, cancer, myocardial contractility disorders, thromboembolism, and atherosclerosis.
PI3K inhibitor compounds and compositions described herein, including salts, metabolites, solvates, solvates of salts, hydrates, and stereoisomeric forms thereof, exhibit anti-proliferative activity and are thus useful to prevent or treat the disorders associated with hyper-proliferation.